Emerging small molecule inhibitors of Bach1 as therapeutic agents: Rationale, recent advances, and future perspectives

The transcription factor Nrf2 is the master regulator of cellular stress response, facilitating the expression of cytoprotective genes, including those responsible for drug detoxification, immunomodulation, and iron metabolism. FDA-approved Nrf2 activators, Tecfidera and Skyclarys for patients with multiple sclerosis and Friedreich's ataxia, respectively, are non-specific alkylating agents exerting side effects. Nrf2 is under feedback regulation through its target gene, transcriptional repressor Bach1. Specifically, in Parkinson's disease and other neurodegenerative diseases with Bach1 dysregulation, excessive Bach1 accumulation interferes with Nrf2 activation. Bach1 is a heme sensor protein, which, upon heme binding, is targeted for proteasomal degradation, relieving the repression of Nrf2 target genes. Ideally, a combination of Nrf2 stabilization and Bach1 inhibition is necessary to achieve the full therapeutic benefits of Nrf2 activation. Here, we discuss recent advances and future perspectives in developing small molecule inhibitors of Bach1, highlighting the significance of the Bach1/Nrf2 signaling pathway as a promising neurotherapeutic strategy.

Harnessing the Therapeutic Potential of the Nrf2/Bach1 Signaling Pathway in Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative movement disorder characterized by a progressive loss of dopaminergic neurons in the substantia nigra pars compacta. Although a complex interplay of multiple environmental and genetic factors has been implicated, the etiology of neuronal death in PD remains unresolved. Various mechanisms of neuronal degeneration in PD have been proposed, including oxidative stress, mitochondrial dysfunction, neuroinflammation, α-synuclein proteostasis, disruption of calcium homeostasis, and other cell death pathways. While many drugs individually targeting these pathways have shown promise in preclinical PD models, this promise has not yet translated into neuroprotective therapies in human PD. This has consequently spurred efforts to identify alternative targets with multipronged therapeutic approaches. A promising therapeutic target that could modulate multiple etiological pathways involves drug-induced activation of a coordinated genetic program regulated by the transcription factor, nuclear factor E2-related factor 2 (Nrf2). Nrf2 regulates the transcription of over 250 genes, creating a multifaceted network that integrates cellular activities by expressing cytoprotective genes, promoting the resolution of inflammation, restoring redox and protein homeostasis, stimulating energy metabolism, and facilitating repair. However, FDA-approved electrophilic Nrf2 activators cause irreversible alkylation of cysteine residues in various cellular proteins resulting in side effects. We propose that the transcriptional repressor of BTB and CNC homology 1 (Bach1), which antagonizes Nrf2, could serve as a promising complementary target for the activation of both Nrf2-dependent and Nrf2-independent neuroprotective pathways. This review presents the current knowledge on the Nrf2/Bach1 signaling pathway, its role in various cellular processes, and the benefits of simultaneously inhibiting Bach1 and stabilizing Nrf2 using non-electrophilic small molecules as a novel therapeutic approach for PD.

An Emerging Role of miRNAs in Neurodegenerative Diseases: Mechanisms and Perspectives on miR146a

Significance: Advancements in and access to health care have led to unprecedented improvements in the quality of life and increased lifespan of human beings in the past century. However, aging is a significant risk factor for neurodegenerative diseases (NDs). Hence, improved life expectancy has led to an increased incidence of NDs. Despite intense research, effective treatments for NDs remain elusive. The future of neurotherapeutics development depends on effective disease modification strategies centered on carefully scrutinized targets. Recent Advances: As a promising new direction, recent evidence has demonstrated that epigenetic processes modify diverse biochemical pathways, including those related to NDs. Small non-coding RNAs, known as microRNAs (miRNAs), are components of the epigenetic system that alter the expression of target genes at the post-transcriptional level. Critical Issues: miRNAs are expressed abundantly in the central nervous system and are critical for the normal functioning and survival of neurons. Here, we review recent advances in elucidating miRNAs' roles in NDs and discuss their potential as therapeutic targets. In particular, neuroinflammation is a major pathological hallmark of NDs and miR146a is a crucial regulator of inflammation. Future Directions: Finally, we explore the possibilities of developing miR146a as a potential biomarker and therapeutic target where additional research may help facilitate the detection and amelioration of neuroinflammation in NDs. Antioxid. Redox Signal. 35, 580-594.

Identification of a potent Nrf2 displacement activator among aspirin-containing prodrugs

Aspirin is a desired leaving group in prodrugs aimed at treatment of neurodegeneration and other conditions. A library of aspirin derivatives of various scaffolds potentially activating Nrf2 has been tested in Neh2-luc reporter assay which screens for direct Nrf2 protein stabilizers working via disruption of Nrf2-Keap1 interaction. Most aspirin prodrugs had a pro-alkylating or pro-oxidant motif in the structure and, therefore, were toxic at high concentrations. However, among the active compounds, we identified a molecule resembling a well-known Nrf2 displacement activator, bis-1,4-(4-methoxybenzenesulfonamidyl) naphthalene (NMBSA). The direct comparison of the newly identified compound with NMBSA and its improved analog in the reporter assay showed no quenching with N-acetyl cysteine, thus pointing to Nrf2 stabilization mechanism without cysteine alkylation. The potency of the newly identified compound in the reporter assay was much stronger than NMBSA, despite its inhibitory action in the commercial fluorescence polarization assay was observed only in the millimolar range. Molecular docking predicted that mono-deacetylation of the novel prodrug should generate a potent displacement activator. The time-course of reporter activation with the novel prodrug had a pronounced lag-period pointing to a plausible intracellular transformation leading to an active product. Treatment of the novel prodrug with blood plasma or cell lysate demonstrated stepwise deacetylation as judge by liquid chromatography-mass spectrometry (LC-MS). Hence, the esterase-catalyzed hydrolysis of the prodrug liberates only acetyl groups from aspirin moiety and generates a potent Nrf2 activator. The discovered mechanism of prodrug activation makes the newly identified compound a promising lead for future optimization studies.

Challenges and Limitations of Targeting the Keap1-Nrf2 Pathway for Neurotherapeutics: Bach1 De-Repression to the Rescue

The Keap1-Nrf2 signaling axis is a validated and promising target for cellular defense and survival pathways. This minireview discusses the potential off-target effects and their impact on future drug development originating from Keap1-targeting small molecules that function as displacement activators of the redox-sensitive transcription factor Nrf2. We argue that small-molecule displacement activators, similarly to electrophiles, will release both Nrf2 and other Keap1 client proteins from the ubiquitin ligase complex. This non-specificity is likely unavoidable and may result in off-target effects during Nrf2 activation by targeting Keap1. The small molecule displacement activators may also target Kelch domains in proteins other than Keap1, causing additional off-target effects unless designed to ensure specificity for the Kelch domain only in Keap1. A potentially promising and alternative therapeutic approach to overcome this non-specificity emerging from targeting Keap1 is to inhibit the Nrf2 repressor Bach1 for constitutive activation of the Nrf2 pathway and bypass the Keap1-Nrf2 complex.

Crosstalk between Nrf2 signaling and mitochondrial function in Parkinson's disease

Search for a definitive cure for neurodegenerative disorders like Parkinson's disease (PD) has met with little success. Mitochondrial dysfunction and elevated oxidative stress precede characteristic loss of dopamine-producing neurons from the midbrain in PD. The majority of PD cases are classified as sporadic (sPD) with an unknown etiology, whereas mutations in a handful of genes cause monogenic form called familial (fPD). Both sPD and fPD is characterized by proteinopathy and mitochondrial dysfunction leading to increased oxidative stress. These pathophysiological mechanisms create a vicious cycle feeding into each other, ultimately tipping the neurons to its demise. Effect of iron accumulation and dopamine oxidation adds an additional dimension to mitochondrial oxidative stress and apoptotic pathways affected. Nrf2 is a redox-sensitive transcription factor which regulates basal as well as inducible expression of antioxidant enzymes and proteins involved in xenobiotic detoxification. Recent advances, however, shows a multifaceted role for Nrf2 in the regulation of genes connected with inflammatory response, metabolic pathways, protein homeostasis, iron management, and mitochondrial bioenergetics. Here we review the role of mitochondria and oxidative stress in the PD etiology and the potential crosstalk between Nrf2 signaling and mitochondrial function in PD. We also make a case for the development of therapeutics that safely activates Nrf2 pathway in halting the progression of neurodegeneration in PD patients.

Hyperhomocysteinemia-induced death of retinal ganglion cells: The role of Müller glial cells and NRF2

Hyperhomocysteinemia (Hhcy), or increased levels of the excitatory amino acid homocysteine (Hcy), is implicated in glaucoma, a disease characterized by increased oxidative stress and loss of retinal ganglion cells (RGCs). Whether Hhcy is causative or merely a biomarker for RGC loss in glaucoma is unknown. Here we analyzed the role of NRF2, a master regulator of the antioxidant response, in Hhcy-induced RGC death in vivo and in vitro. By crossing Nrf2^−/− mice and two mouse models of chronic Hhcy (Cbs^+/- and Mthfr^+/- mice), we generated Cbs^+/-Nrf2^−/− and Mthfr^+/-Nrf2^−/− mice and performed systematic analysis of retinal architecture and visual acuity followed by assessment of retinal morphometry and gliosis. We observed significant reduction of inner retinal layer thickness and reduced visual acuity in Hhcy mice lacking NRF2. These functional deficits were accompanied by fewer RGCs and increased gliosis. Given the key role of Müller glial cells in maintaining RGCs, we established an ex-vivo indirect co-culture system using primary RGCs and Müller cells. Hhcy-exposure decreased RGC viability, which was abrogated when cells were indirectly cultured with wildtype (WT) Müller cells, but not with Nrf2^−/− Müller cells. Exposure of WT Müller cells to Hhcy yielded a robust mitochondrial and glycolytic response, which was not observed in Nrf2^−/− Müller cells. Taken together, the in vivo and in vitro data suggest that deleterious effects of Hhcy on RGCs are likely dependent upon the health of retinal glial cells and the availability of an intact retinal antioxidant response mechanism.

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Oxidative stress is linked to homocysteine (Hcy)-induced retinal ganglion cell death.

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NRF2's role in protecting ganglion cells from excess Hcy was studied in vitro/vivo.

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Hyper-Hcy mice were crossed with Nrf2^−/− mice to study retinal function/structure.

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Ganglion cells co-cultured with primary WT Müller glial cells survived Hcy treatment.

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Nrf2^−/− Müller cells did not afford neuroprotective advantage to Hcy-treated cells.

Excess homocysteine upregulates the NRF2-antioxidant pathway in retinal Müller glial cells

This study evaluated the effects of elevated homocysteine (Hcy) on the oxidative stress response in retinal Müller glial cells. Elevated Hcy has been implicated in retinal diseases including glaucoma and optic neuropathy, which are characterized by retinal ganglion cell (RGC) loss. To understand the mechanisms of Hcy-induced RGC loss, in vitro and in vivo models have been utilized. In vitro isolated RGCs are quite sensitive to elevated Hcy levels, while in vivo murine models of hyperhomocysteinemia (HHcy) demonstrate a more modest RGC loss (∼20%) over a period of many months. This differential response to Hcy between isolated cells and the intact retina suggests that the retinal milieu invokes mechanisms that buffer excess Hcy. Oxidative stress has been implicated as a mechanism of Hcy-induced neuron loss and NRF2 is a transcription factor that plays a major role in regulating cytoprotective responses to oxidative stress. In the present study we investigated whether HHcy upregulates NRF2-mediated stress responses in Müller cells, the chief retinal glial cell responsible for providing trophic support to retinal neurons. Primary Müller cells were exposed to L-Hcy-thiolactone [50μM-10mM] and assessed for viability, reactive oxygen species (ROS), and glutathione (GSH) levels. Gene/protein levels of Nrf2 and levels of NRF2-regulated antioxidants (NQO1, CAT, SOD2, HMOX1, GPX1) were assessed in Hcy-exposed Müller cells. Unlike isolated RGCs, isolated Müller cells are viable over a wide range of Hcy concentrations [50 μM - 1 mM]. Moreover, when exposed to elevated Hcy, Müller cells demonstrate decreased oxidative stress and decreased ROS levels. GSH levels increased by ∼20% within 24 h exposure to Hcy. Molecular analyses revealed 2-fold increase in Nrf2 expression. Expression of antioxidant genes Nqo1, Cat, Sod2, Hmox1, Gpx1 increased significantly. The consequences of Hcy exposure were evaluated also in Müller cells harvested from Nrf2^-/- mice. In contrast to WT Müller cells, in which oxidative stress decreased upon exposure to Hcy, the Nrf2^-/- Müller cells showed a significant increase in oxidative stress. Our data suggest that at least during early stages of Hhcy, a cytoprotective response may be in place, mediated in part by NRF2 in Müller cells.

Current perspective of mitochondrial biology in Parkinson's disease

Parkinson's disease (PD) is one of the most common neurodegenerative movement disorder characterized by preferential loss of dopaminergic neurons of the substantia nigra pars compacta and the presence of Lewy bodies containing α-synuclein. Although the cause of PD remains elusive, remarkable advances have been made in understanding the possible causative mechanisms of PD pathogenesis. An explosion of discoveries during the past two decades has led to the identification of several autosomal dominant and recessive genes that cause familial forms of PD. The investigations of these familial PD gene products have shed considerable insights into the molecular pathogenesis of the more common sporadic PD. A growing body of evidence suggests that the etiology of PD is multifactorial and involves a complex interplay between genetic and environmental factors. Substantial evidence from human tissues, genetic and toxin-induced animal and cellular models indicates that mitochondrial dysfunction plays a central role in the pathophysiology of PD. Deficits in mitochondrial functions due to bioenergetics defects, alterations in the mitochondrial DNA, generation of reactive oxygen species, aberrant calcium homeostasis, and anomalies in mitochondrial dynamics and quality control are implicated in the underlying mechanisms of neuronal cell death in PD. In this review, we discuss how familial PD-linked genes and environmental factors interface the pathways regulating mitochondrial functions and thereby potentially converge both familial and sporadic PD at the level of mitochondrial integrity. We also provide an overview of the status of therapeutic strategies targeting mitochondrial dysfunction in PD. Unraveling potential pathways that influence mitochondrial homeostasis in PD may hold the key to therapeutic intervention for this debilitating neurodegenerative movement disorder.

Bioactive Flavonoids and Catechols as Hif1 and Nrf2 Protein Stabilizers - Implications for Parkinson's Disease

Flavonoids are known to trigger the intrinsic genetic adaptive programs to hypoxic or oxidative stress via estrogen receptor engagement or upstream kinase activation. To reveal specific structural requirements for direct stabilization of the transcription factors responsible for triggering the antihypoxic and antioxidant programs, we studied flavones, isoflavones and catechols including dihydroxybenzoate, didox, levodopa, and nordihydroguaiaretic acid (NDGA), using novel luciferase-based reporters specific for the first step in HIF1 or Nrf2 protein stabilization. Distinct structural requirements for either transcription factor stabilization have been found: as expected, these requirements for activation of HIF ODD-luc reporter correlate with in silico binding to HIF prolyl hydroxylase. By contrast, stabilization of Nrf2 requires the presence of 3,4-dihydroxy- (catechol) groups. Thus, only some but not all flavonoids are direct activators of the hypoxic and antioxidant genetic programs. NDGA from the Creosote bush resembles the best flavonoids in their ability to directly stabilize HIF1 and Nrf2 and is superior with respect to LOX inhibition thus favoring this compound over others. Given much higher bioavailability and stability of NDGA than any flavonoid, NDGA has been tested in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-animal model of Parkinson's Disease and demonstrated neuroprotective effects.

Diisopropylfluorophosphate Impairs the Transport of Membrane-Bound Organelles in Rat Cortical Axons

The extensive use of organophosphates (OPs) is an ongoing environmental health concern due to multiple reports of OP-related neurologic abnormalities. The mechanism of the acute toxicity of OPs has been attributed to inhibition of acetylcholinesterase (AChE), but there is growing evidence that this may not account for all the long-term neurotoxic effects of OPs. In previous experiments (using ex vivo and in vitro model systems) we observed that the insecticide OP chlorpyrifos impaired the movements of vesicles and mitochondria in axons. Here, using a time-lapse imaging technique, we evaluated the OP-nerve agent diisopropylfluorophosphate (DFP) across a wide range of concentrations (subnanomolar to micromolar) for effects on fast axonal transport of membrane-bound organelles (MBOs) that contain the amyloid precursor protein (APP) tagged with the fluorescent marker Dendra2 (APPDendra2). Both 1 and 24 hours of exposure to DFP and a positive control compound, colchicine, resulted in a decrease in the velocity of anterograde and retrograde movements of MBOs and an increase in the number of stationary MBOs. These effects occurred at picomolar (100 pM) to low nanomolar (0.1 nM) concentrations that were not associated with compromised cell viability or cytoskeletal damage. Moreover, the effects of DFP on axonal transport occurred at concentrations that did not inhibit AChE activity, and they were not blocked by cholinergic receptor antagonists. Given the fundamental importance of axonal transport to neuronal function, these observations may explain some of the long-term neurologic deficits that have been observed in humans who have been exposed to OPs.

Methylene blue upregulates Nrf2/ARE genes and prevents tau-related neurotoxicity

Methylene blue (MB, methylthioninium chloride) is a phenothiazine that crosses the blood brain barrier and acts as a redox cycler. Among its beneficial properties are its abilities to act as an antioxidant, to reduce tau protein aggregation and to improve energy metabolism. These actions are of particular interest for the treatment of neurodegenerative diseases with tau protein aggregates known as tauopathies. The present study examined the effects of MB in the P301S mouse model of tauopathy. Both 4 mg/kg MB (low dose) and 40 mg/kg MB (high dose) were administered in the diet ad libitum from 1 to 10 months of age. We assessed behavior, tau pathology, oxidative damage, inflammation and numbers of mitochondria. MB improved the behavioral abnormalities and reduced tau pathology, inflammation and oxidative damage in the P301S mice. These beneficial effects were associated with increased expression of genes regulated by NF-E2-related factor 2 (Nrf2)/antioxidant response element (ARE), which play an important role in antioxidant defenses, preventing protein aggregation, and reducing inflammation. The activation of Nrf2/ARE genes is neuroprotective in other transgenic mouse models of neurodegenerative diseases and it appears to be an important mediator of the neuroprotective effects of MB in P301S mice. Moreover, we used Nrf2 knock out fibroblasts to show that the upregulation of Nrf2/ARE genes by MB is Nrf2 dependent and not due to secondary effects of the compound. These findings provide further evidence that MB has important neuroprotective effects that may be beneficial in the treatment of human neurodegenerative diseases with tau pathology.

Epigenetic landscape of Parkinson's disease: emerging role in disease mechanisms and therapeutic modalities

Parkinson's disease (PD) is a complex multifactorial disorder marked by extensive system-wide pathology, including a substantial loss of nigrostriatal dopaminergic neurons. The etiology of PD remains elusive, but there is considerable evidence that, in addition to well-defined genetic mechanisms environmental factors play a crucial role in disease pathogenesis. How the environment might influence the genetic factors and contribute to disease development and progression remains unclear. In recent years, epigenetic mechanisms such as DNA methylation, chromatin remodeling and alterations in gene expression via non-coding RNAs have begun to be revealed as potential factors in PD pathogenesis. Epigenetic modulation exists throughout life, beginning in prenatal stages, is dependent on the lifestyle, environmental exposure and genetic makeup of an individual and may serve as a missing link between PD risk factors and development of the disease. This chapter sheds light on the emerging role of epigenetics in disease pathogenesis and on prospective interventional strategies for the therapeutic modulation of PD.

Targeting Nrf2-mediated gene transcription by extremely potent synthetic triterpenoids attenuate dopaminergic neurotoxicity in the MPTP mouse model of Parkinson's disease

Although the etiology of Parkinson's disease (PD) remains unclear, ample empirical evidence suggests that oxidative stress is a major player in the development of PD and in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity. Nuclear factor E2-related factor 2 (Nrf2) is a redox-sensitive transcription factor that upregulates a battery of antioxidant response element (ARE)-driven antioxidative and cytoprotective genes that defend against oxidative stress.

AIMS

We evaluated whether the strategy of activation of Nrf2 and its downstream network of cytoprotective genes with small molecule synthetic triterpenoids (TP) attenuate MPTP-induced PD in mice.

RESULTS

We show that synthetic TP are thus far the most potent and direct activators of the Nrf2 pathway using a novel Neh2-luciferase reporter. They upregulate several cytoprotective genes, including those involved in glutathione biosynthesis in vitro. Oral administration of TP that were structurally modified to penetrate the brain-induced messenger RNA and protein levels for a battery of Nrf2-dependent cytoprotective genes reduced MPTP-induced oxidative stress and inflammation, and ameliorated dopaminergic neurotoxicity in mice. The neuroprotective effect of these TP against MPTP neurotoxicity was dependent on Nrf2, since treatment with TP in Nrf2 knockout mice failed to block against MPTP neurotoxicity and induce Nrf2-dependent cytoprotective genes.

INNOVATION

Extremely potent synthetic TP that are direct activators of the Nrf2 pathway block dopaminergic neurodegeneration in the MPTP mouse model of PD.

CONCLUSION

Our results indicate that activation of Nrf2/antioxidant response element (ARE) signaling by synthetic TP is directly associated with their neuroprotective effects against MPTP neurotoxicity and suggest that targeting the Nrf2/ARE pathway is a promising approach for therapeutic intervention in PD.